The structure of cytochrome oxidase membranes

Abstract

Two preparations of oxidized membranous cytochrome oxidase were examined using both the negative stain and freeze fracture techniques of electron microscopy. The cholate preparation, containing 27% phospholipid by weight, appeared as membrane sheets. With the freeze fracture technique, cross fractures of the sheets showed particles abutted one against another. The Triton preparation, containing 42% phospholipid by weight, was decidedly vesicular and only particulate membrane fracture surfaces were usually seen. In the case of membranous cytochrome oxidase it is proposed that fractures follow the surface rather than a centre plane through the hydrophobic interior of the membrane. The observed particles appeared as prolate spheroids of 135 A X 68 A with the long axis extended through the membrane. From the size shape relationships, a molecular weight of 244 000 was calculated which is in agreement with values reported for dispersed cytochrome oxidase using other physical techniques. At least 50% of the protein is exposed to the aqueous environment. Cytochrome oxidase is a lipoprotein that spontaneously forms membranes under conditions of low ionic strength and the absence of detergents. Sun et al. [ 1 ] showed that these membranes were trilaminate structures 55 A thick when examined by conventional thin section techniques. Negatively stained preparations showed a membrane surface covered with 50 A particles. Seki and Oda [2] have shown